Using isotope-tracer method with deuterium for quantitative study of hydrogen activation mechanism in direct coal liquefaction

Understanding hydrogen activation is crucial for improving the performance of direct coal liquefaction (DCL) which is essential for the clean, efficient conversion of coal to fuel oils and aromatic chemicals. Three hydrogen sources are involved in DCL: solvent hydrogen (SH), dissolved hydrogen that reacts directly (DHD), and dissolved hydrogen that reacts through a solvent (DHS). Quantitatively studying DHS is challenging because it is generated through the dehydrogenation of hydrogen-donor solvents produced via the hydrogenation of gas-phase H2 and solvents. This research establishes for the first time a quantitative method for determining DHS consumption (DHSC) based on protium- and deuterium-nuclear-magnetic-resonance results for solvents after isotope-tracer reactions. Comparative experiments and quantum-chemical calculations were performed to confirm the method's reliability. The isotope-tracer method showed that DHSC accounts for < 7 % of the total hydrogen consumption under catalysis with ferric stearate, nickel stearate, or molybdenum 2-ethylhexanoate, while DHD consumption accounts for > 50 %. Thus, DHD, rather than DHS, is the primary hydrogen source for catalytic activation. Furthermore, the comparative experiments also showed that hydrogen consumption is greater for one hydrogen source than for the coexistence of the three hydrogen sources, indicating competition among the three sources. The quantum-chemical calculations showed that the competitiveness among the three sources follows the order of DHS < SH < DHD, this agrees with the order of hydrogen consumption in the isotope-tracer experiments. This study quantitatively reveals the mechanism responsible for hydrogen activation by catalysts and provides a scientific basis for optimization and mutual matching of solvents and catalysts.